Balllistics Program Question

Let's say that on the day you decide to zero your scope, the following conditions exist: 300 ft. altitude, 48 deg.(F), 30.10 in.Hg., 65% humidity. You zero at 100 yds.

Two weeks later, you take a shot at a target 1000 yds. away. On that day, you're at 2500 ft., 65 deg. (F), 29.54 in.Hg., 74% hum. You happen to have a PDA with the Horus ballistics software and punch in the atmospheric data, crank in the elevation indicated and take the shot. You are way high.

The problem is that the program (and every other external ballistics program I've seen) assumes that you've zeroed under the current conditions and the scope data is from that current zero. The program doesn't know that you've zeroed under different atmospheric conditions. What is needed, therefore, is a an offset to move the zero to current conditions. It seems to me that this offset will apply uniformly to the new data. In other words, let's say that a 100 yd. shot under current conditions with the earlier zero would wind-up with a hit .75 MOA high at 100 yds. It seems to me that all current data would have to be adjusted by subtracting this offset from all elevation data in order to correct for the zero obtained under different conditions.

So, first: is this right?

Second: if it is, what is the easiest way to compute this offset? (I need a formula here.)

Atmospherics at 100 yards are probably close to a non-issue. (There seems to be something else at work at 100 yards when changing geographic locations though, Len and I have discussed this 100 yard zero problem before.)

In tests I performed in Colorado (change from 600 ASL to 10,500 ASL) I didn't see much difference in trajectories until about 400 yards. (Shot with 600 ASL data at 10,500 ASL just to check elevation change.)

In the TRGT Data Book there is a Zero Data Summary page that allow a shooter to track data at many ranges and temperatures. A fella could easily make one for other atmospheric and geographic conditions.

S1

I don't believe your supplied formula is correct as written. There seems to be a method to remove the 100 yard zero data from the "old" conditions but no place to reintroduce the corrected "new" 100 yard data.

I think you're right. From S1's post, I decided to look at bullet drop which is simply a function of time. For a 178 gr. in 308 at 2675 at the muzzle, total bullet drop for the 100 yd. trip was 2.55 inches. Elevating the temp and lowering the pressure significantly lowered the drop to 2.54 inches, an insignificant amount. So it would appear that once zeroed at 100 yds., any change of atmospherics would have no real effect on that zero.

Of course, if the initial zero was at 600 yds., a correction factor would be required and using bullet drop over time would seem to be the approach to computing this factor.

I don't know if this is the same, I thought all the BC's were from Standard Conditions, Sealevel-59 Deg.-BP 29.53-RH 78%. Then changing conditions basicly changed your BC?

Also do velosity change with conditions (temp sensitive powder)and unless you shoot a crony how to know without actual testing?

All ballistic programs need MV is the MV the same at sealevel as at 10,000 ft? How do you guys know what MV to use with out actual testing at different elev? say you have 3450 FPS at 5000 ft Elev. and you plan to hunt at 10,000 ft elev

What form or info do you record at the test range? ( temp, Vel, RH, BP, sunny, etc. )
Is there a good form to print out for all the info?

Also do velosity change with conditions (temp sensitive powder)and unless you shoot a crony how to know without actual testing?

You don't...

Recording the actual velocity drop at different temps and adjusting the MV input on the program is the only way to be sure and accuraate. This will affect the drop more than the density change temp variations cause, in most cases unless the powder used is very temp insensitive.

Temperature sensitive powders are a bit of a problem, try to stay away from the worst or operate with that understanding.

Elevation and other atmospherics (excluding temperature sensitive powders) will have little or no effect on Muzzle Velocity (MV), it will effect the downrange trajectory by increasing or decreasing the flight time for a given (similar) distance.

I use the same MV at any altitude, I do verify long range trajectory prior to a shoot at different elevations (significant elevation changes). I have a ballistic prediction before entering the new area and verifiy the data then make a comparison with the "home" (in my case, flat land) data.

I print my own "zero" targets in Microsoft Word. Use the grid feature and set the grid to 1", add a header and footer with the data you feel is important. I include, distance, date, time, temp, wind speed, powded name and weight, bullet name and weight, rifle, cartridge (including brass name and prep), primer, trigger weight, scope name and setting. I have a wind and light "circles". On the side I have numbered lines for the load (shot) number and MV as well as my "call" info. I use these targets in paired sets, one goes downrange to the backer and the other stays on the firing line with the rifle. I plot and number the impact position of the rounds as they are fired, I later use this with the MV to find a "sweet spot".

Standard printer paper is a little fragile and the holes sometimes rip, I use clear packing tape on the back of the downrange target to make them more durable (a needed additive).